A condition or state (term is based on the name of the UK group "Fatboy Slim") wherein:

1) A fat girl eats like a bird in public, thereby suggesting that she is fat NOT because she eats everything in sight, but because she suffers from some kind of thyroidal dysfunction
2) A fat girl has lost weight & yet managed to retain good qualities developed from when she was a fat girl (such as her low-maintenance personality or sense of humor)

Girl 1: You think Maggie really has a thyroid problem? Whenever we get lunch she eats like, half her sandwich and drinks Coke Light..
Girl 2: Yeah, no. The guys left a hidden camera in that classroom that no 1 ever uses & caught her wolfing down 3 Big Macs after school hours.
Girl 1: So she's a run-of-the-mill fatgirl slim..